Fix PG::GroupingError with label filters on MR analytics page

What does this MR do?

This MR fixes the PG::GroupingError on the MR analytics page when more than 1 label filters are given.

Note: this is only happening when the optimized_issuable_label_filter feature flag is off. The flag is currently disabled by default.

How to reproduce it:

1. Make sure you have ultimate license
2. Make sure your project has at least two labels
3. Go to a project and open the MR analytics page via the sidebar: Analytics > Merge request
4. Filter by two labels
5. See the 500 errors in the inspector (GraphQL request)

The fix should return 200 (success) response.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #280535 (closed)